1. Field of the Invention
The present invention relates to a distributed feedback laser apparatus, and more particularly to a distributed feedback laser apparatus that produces a dithered optical signal having a frequency versus time characteristic that is represented by a triangular waveform.
2. Discussion of the Prior Art
In high power coherent arrays of fiber amplifiers, a single laser source seeds all the fiber amplifiers. The source must provide a coherent common temporal coherence to all the fiber amplifiers in the array. However, coherent high power fiber arrays are limited in power by nonlinear processes that erode the mutual output phase coherence relative to that of the common seed source. The primary optical phase distortion mechanisms in these fiber amplifiers include four-wave mixing and cross phase modulation. These mechanisms can be suppressed by employing a single frequency laser source. However, another nonlinear optical interaction due to stimulated Brillouin scattering (SBS) prevents fiber amplifiers from reaching high power output.
SBS is an inherent effect that occurs in fiber amplifiers in which a substantial fraction of the forward-propagating power in the amplifier is converted into backward propagating power with a slight downward frequency shift. This limits the power transfer through a fiber amplifier. SBS gain is dependent on beam intensity, spectral width, and fiber length. It should be recognized that SBS does not generally occur at low powers and for short lengths of optical fiber over which a signal is transmitted.
SBS has occasionally been a problem for telecommunications applications, but has been mitigated by broadening the spectrum of the signal. This is accomplished by passing a narrow line signal from a distributed feedback source through a modulator to effectively broaden the linewidth of the source. This raises the SBS threshold and thus avoids the creation of SBS. This type of source, however, is only available in very low power communications where the optical power is less than about 1 watt and the optical fiber is about 1 kilometer or greater. In these low power applications, repeaters are necessary along the optical fiber to amplify the optical signals passing therethrough.
An example of a semiconductor laser for use in suppressing SBS in a passive fiber in optical communications is found in U.S. Pat. No. 5,473,625 entitled xe2x80x9cTunable Distributed Bragg Reflector Laser for Wavelength Ditheringxe2x80x9d by Hansen et al. As will be described it employs a sinusoidal dither that has too much dwell time in the peak and valleys of the sinusoid, thus allowing SBS to occur.
What is needed, therefore, is a seed laser system that eliminates phase fidelity degradation due to four-wave mixing and produces an optical signal whose frequency is varied so rapidly that the medium cannot respond in a manner to reach SBS threshold in a fiber amplifier of given length and power.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides a seed laser system. The system includes a current source for supplying a current signal and a distributed feedback laser that responds to the current signal and transmits a dithered optical signal having a frequency versus time characteristic that is represented in its most general form by an asymmetric triangular form. An optical medium includes a plurality of optical paths each having an optical fiber, a phase modulator and a fiber amplifier, and is characterized by a response time associated with stimulated Brillouin scattering (SBS). The height of the triangular waveform is related to the SBS response time and amplifier parameters. The period of the waveform is shorter than the response time of the SBS or equal to the round trip optical transmit time in the fiber. Preferably the waveform is of the sawtooth type.